In heart muscle cells, the contractile units must be linked by Z bands for effective contraction. The Z band is a complex protein lattice which is an integral part of the contractile apparatus, but its exact role in contraction is not known. Z bands vary in width in normal and diseased muscles, but the functional significance of this variation remains obscure. Using optical diffraction methods, we have identified a common structural unit in Z bands of mammalian cardiac and slow skeletal muscle. Our optical reconstruction studies show a regular arrangement of axial and cross connecting filaments. However, the Z band is rarely a uniform lattice, because there is variation in the arrangement of cross connecting filaments. Two different lattice forms, basket weave and small square are observed and suggest that the Z band is a dynamic structure capable of responding to different physiological states of the muscle. Our purpose in these studies is to examine structural differences in the Z band lattice between cardiac and slow skeletal muscle, between slow and fast skeletal muscle, between adult and developing muscle, between normal and diseased muscle and to correlate them with specific structural states induced experimentally. We will generate a three dimensional reconstruction of the Z band by computerized image processing of a tilt series of electron micrographs. Then we can determine the impact of specific rearrangements within the lattice in different muscle types in different physiological states. Using goniometer tilt and stereo techniques at standard and high voltages, together with optical diffraction and reconstruction methods, we will examine Z bands 1) in normal muscles fixed or frozen before, during, and after tetanic contraction; 2) in normal muscle partially extracted and reconstituted; 3) in isolated myofibrils; and 4) in postnatally developing intact muscle. A systematic analysis will be used to answer two questions. Why does the lattice move? How is the Z band formed? The emphasis will be on the three dimensional structure of the Z band lattice. New information about Z bands structure will help us to understand the significance of widened Z bands in developing muscle and in diseased muscle and may suggest a mechanism for the formation and breakdown of the Z bands.